The invention relates to a rotary drill bit for deephole drilling in subsurface earth formations, and in particular to a drill bit including a bit body which is suitable to be coupled to the lower end of a drill string and carries a plurality of cutting elements.
Bits of this type are known and disclosed, for example, in U.S. Pat. Nos. 4,098,362 and 4,244,432. The cutting elements of the bits disclosed in these patents are preformed cutters in the form of cylinders that are secured to the bit body either by mounting the elements in recesses in the body or by brazing or soldering each element to a pin which is fitted into a recess in the bit body. The impacts exerted to the cutting elements during drilling are severe and in order to avoid undue stresses in the elements, the frontal surface of each element is generally oriented at a negative top rake angle between zero and 20 degrees.
The cutting elements usually comprise an abrasive front layer consisting of synthetic diamonds or cubic boron nitride particles that are bonded together to a compact polycrystalline mass. The front layer of each cutting element may be backed by a cemented tungsten carbide substratum to take the thrust imposed on the front layer during drilling. Preformed cutting elements of this type are disclosed in U. S. Pat. No. 4,194,790 and in European patent specification No. 0029187 and they are often indicated as composite compact cutters, or--in case the abrasive particles are diamonds--as polycrystalline diamond compacts (PDC's).
The cutting elements of bits of the above type are usually provided with an abrasive front layer of which the thickness is selected such that a compromise is accomplished between various desired drilling characteristics.
For example, a low thickness of the abrasive front layer provides a cutting element that remains comparatively sharp throughout its life, so that a high bit agressiveness level (defined as the ratio between bit torque and weight on bit) is achieved. However, a high bit agressiveness level has the consequence that in certain formations the stalling tendency of the bit as a result of weight on bit fluctuations is high. In particular, if the bit is driven by a downhole drive such as a mud driven turbine, this stalling tendency may result in continuous fluctuations of the bit rotation rate and a poor drilling progress.